Bone mineral density (BMD) is highly correlated with body weight, and weight loss is associated with reduced BMD. Whether such losses of BMD increase skeletal fragility is unclear. We examined the effect of 9 wk of energy restriction (ER) on bone density, mineral and matrix protein composition and biomechanical properties in mature (20 wk old, n = 12) and aged (48 wk old, n = 16) female rats. Energy-restricted rats were fed 40% less energy than controls that consumed food ad libitum. Bone content of mineral (ash and calcium content) and matrix proteins (hydroxyproline, pyridinium crosslinks and proteoglycans), serum hormones, site-specific bone density and biomechanical properties (peak load, peak torque, shear stiffness and bending stiffness) were measured at the conclusion of the study. In both age groups, ER reduced body weight by 15 +/- 10% (P < 0.001) and dramatically decreased femoral bone density by 32-35% (P < 0.01) compared with controls. Energy restriction resulted in a small reduction in tibia and humerus density, as well as biomechanical properties in the aged but not mature rats (P < 0.05). Reduced serum levels of insulin and estradiol due to ER in aged rats (P < 0.05) may play a role in altering bone quality. These data show that although weight loss due to ER is detrimental to some bone parameters in mature rats, only aged rats show consistent reductions in bone density and biomechanical properties.